RNA polymerase III is a large protein complex involved in the transcription of tRNA and 5S RNA. The protein complex was affinity-purified from protein extract of a cell line which constitutively expresses an epitope-tagged subunit of human RNA polymerase III. We are using MALDI-ion trap mass spectrometer and MALDI-TOF mass spectrometer to verify the known components of RNA polymerase III and identify novel subunits.